In my daily use of linux, I've come to like few softwares over years that I believe serves a good replacement for their paid alternatives. Here are the following:

A torrent client with an in-built search engine! There are more of them but the others all suck. For example, Vuze has ads in its free version. Tixati is bloated to hell and back. Frostwire is also bloated - video players have no place inside torrent clients. KTorrent is the only other one that's open source but the last time I used it, most of its search engines didn't work and displayed a big ugly error. That, then, leaves qBittorrent, which also has a really nice, clean user interface (though an unfortunate qt dependency).
Even better than torrents. As qB, it has a search engine, but is also more configurable in other ways. You can add friends and send messages, but the most important feature is the ability to set a SOCKS proxy server - yes, this means you can download files through TOR. In an age where ISPs literally send letters trying to scare you from torrenting, and some people even get fined, this is very important. Note: I wasn't able to get High ID with TOR - it doesn't seem to be possible, and apparently is with some VPNs. However (most) VPNs are paid, and can rat you out. Also, only TCP traffic goes through TOR, while aMule still sends some UDP traffic, but the actual file is downloaded through TOR - I know because when I turned it off, the download stopped. I think the UDP is just for server requests. Regardless, this is uncharted territory - not much information at all on running aMule through TOR, so some issues could still pop up. TOR does not provide perfect security regardless - even with all the effort spent on the TOR browser, they can barely deal with simple things such as fingerprinting. TOR itself was deanonymized many times, and probably many more tactics are in preparation or even active, but unknown to the public. But it is better than relying on direct connections, and is your best free option. Works with GTK2.The best Linux music player! (and I think I've tried them all). The UI is great. Everything is so intuitive and usable. You can choose what you see on the sidebar - playlists, album lists, your folders, etc. Its preferences menu has everything it needs to and nothing more (while some other players, like DeadBeeF or Clementine, have too much stuff shoved in there). In the track view, you can easily add or remove the visible columns that you need - and there are LOTS of them, while DeadBeeF only has a few. Supports ratings and grouping tags (which can also be displayed as columns in the song list) - and here we come to QuodLibet's most important feature, which is missing in other music players. The advanced tag filtering capabilities. Every option that is available as a column can be used as a search term. So you can search by length, album, author, whatever. And combine all those in any way you want. Let's say I'm not in the mood for short or sad songs and I've assigned those a relevant grouping value. So I can type (grouping = !sad, grouping = !short), which means "show me songs where the grouping tag is NEITHER sad nor short". I also don't want any badly rated songs, so let's add a rating over 0.5. &(grouping = !sad, grouping = !short, #(rating > 0.5)). See? You can do everything with this, and no other player has this capability. QuodLibet 2.4.1 works with GTK2 and doesn't appear to be missing any relevant features despite being 11 years old. The only player other than QuodLibet that has the regex search is cmus - but it is console-based and requires taking systemdicks up your ass elogind.

A mail client that (similar to SpaceFM) manages to be featureful while not getting in your way. UI is clean and immediately understandable; I especially like the Preferences menu where you can see all the categories at once on the left side and the options they consist of on the right side. Using Claws Mail has been painless and very efficient for me - you can easily do literally everything you want in regards to mail handling (PGP support available through plugin - works very well) - while being able to ignore the things you don't care about without hassle. More importantly - when the security of various mail clients was tested (against the EFAIL attack), only Claws and Mutt passed them with flying colors:

 

Contrast that to Thunderbird (usually the E-mail software of choice for Linux systems) which was one of the only clients vulnerable to all three of the above attacks (two of them without user interaction). Then again, it is not even a mail client but an abomination consisting of an RSS reader, calendar, task manager, and a fucking chat program. It even has a default search engine and fucking tabs - like what the fuck? This isn't a web browser! Of course, the UI is also terrible - check out Principles of bad software design to know more. There is also Sylpheed (from which Claws Mail has been forked a long time ago) which is a decent choice but lacks some important features while not really being more usable. The Preferences menu is badly designed as well. Important privacy warning: Claws Mail reveals your timezone by default (in the headers of sent E-mails), which could be sensitive especially if you have a rare one like +12 or such. To remove that "feature", go to /home/username/.claws-mail/ and open the file clawsrc. In it, find the line hide_timezone and change the value to 1. Works with GTK2.

An RSS client joining the list of lean, simple and solid software with just enough features to be useful without overburdening the user. But first - what are RSS clients and why use them? Briefly - some websites choose to supply a document called an RSS (or Atom) feed which contains all the articles / news it has submitted. A client can then mass download those with one click, save them and read them locally. It is a great debloater of the Web, displaying all the headlines at once while avoiding the fluff. You can then quickly choose what interests you and read it in Liferea's simple viewer (there's a gimped version of the article sometimes, though - depending on what the site has chosen) or go to the site directly. Liferea's features are exactly what's needed and no more - the ability to group feeds by folders, marking them so they avoid being automatically dumped, searching, exporting the feed list, offline mode, auto-detection of a site's RSS feed location...It's the perfect combination of usefulness and simplicity. Contrast with RSSOwl which has way too much shit integrated into it. Anyway, I really recommend RSS clients in general - you don't know what you're missing until you try them, like with mail clients versus webmail. Another advantage of RSS is being able to push / view all the updates since you've last read a site. Really handy for both web developers and readers! There are not many RSS clients at all - pretty much just RSSOwl (bloat), Thunderbird (mozillaware), QuiteRSS (bloated qt dependency...) and some console-only ones - so Liferea is the recommendation. UPDATE: the latest version I can find that supports GTK2 is 1.8.7. It works perfectly fine.
  • Other GTK2 software

PeaZip or xarchiver for archive management. Older versions of Transmission for torrenting, alternatively rtorrent. MPlayer is the only video player as far as I can see, and it lacks any actual user interface. Atril for PDF viewing. Veracrypt works for disk encryption. The only "modern" GTK2 browser is Pale Moon. Geany IDE requires an actual review, but for now putting it in here. Pidgin or old versions of Gajim as XMPP clients; alternatively Profanity. Ristretto image viewer (edit: actually, Mirage is better, with more features especially the default zoom). All system settings, etc. tools from Xfce 4.12 work.