Retro Achievements is a project very dear to my heart. I've grown up with (a schoolmate's) SNES and been playing old games (mostly the SNES games I knew from my childhood) ever since I first downloaded an emulator (around 2004 I would guess). RA provides exactly what the name says. You simply register a RA account, download the modified emulators from there and the roms from any rom site and BAM you can play retrogames with achievements. Nintendo and Sega consoles and handhelds are available.
I found the site (by random googling) when there were 1000 members or so (some 3 years ago) and now it has over 10000, so it's growing nicely. I've entered the top 500 today, so still a way to go - I doubt I have the potential to become #1 since I never was one to quickly learn and master new games (or even play them for more than 2 hours if I didn't immediately figure them out) after I entered my teenage years (and especially during the last 5 or so years where I barely played anything new), but that's not much of a concern to me now. My only hope of ever getting to a competitive level in a multiplayer game died with League of Legends (which I am still sad about, though I don't miss it anymore - in fact I can barely remember ever playing it), so now the only thing I can excel at is to reach world class in certain single-player retrogames.
Back when I joined RA, I introduced the concept of a hardcore mode (i.e. no save-stating) to the administrator of RA, and he actually implemented it! Certainly a rare and blissful moment of being able to convince somebody on the internet of doing the logical thing. ^^
Anyway, my most important accomplishments so far are:
Grandmastery (i.e. getting all achievements for a game in hardcore - by the way, I only play hardcore! achieved in the order listed below):
-) Super Mario World
-) Donkey Kong Country 3
-) Kirby's Pinball Land
-) Donkey Kong Country 2
-) Donkey Kong Country 1
Achievements that were specifically hard to get included beating Lost World K.Rool in DKC3 and King K.Rool in DKC2 while taking no hit in the respective battle - I had massive trouble with both of those and actually consider the first-mentioned the hardest thing I've ever done in a (single-player) video game.
Other noticeable events included me beating a Pokemon game for the first time in my life (on emulator at least, I probably beat Pokemon Red or Blue on a schoolmate's gameboy back when it came out) and scoring the highest legitimate score on Kirby Pinball I could find documented video-wise on the internet - one guy has over 17 million but himself basically admitted that score had been scored with cheating). Sure, that game can be cheesed pretty easily, but it still takes a lot of effort to stay concentrated during the many hours (pinball is like that) it takes to score big. Digital pinball games are the video game genre I'm absolutely best at, and I hold some de facto world records (even though of course competition on these games was never that high - but still!)
So, more thoughts on video games when I full clear some more games. Right now I can't really see what the next one will be though...