Preferably platform/action-adventure type. Something along the lines of Cave Story or I Wanna Be The Guy but NOT as hard as IWBTG. I hurt my thumb playing that awhile back and still only got about halfway through it before giving up for the sake of my sanity.
Waddayagot?
Spelunky is amazing. It isn’t hard because dying is fun!
Jumpman is quite good - and it has a great 8-bit feel. (Note: Not browser-based, small download).
It’s not freeware, but the entire commander keen series is on Steam for $6.
It’s not adventure, but Death Worm was fun for a while. I don’t vouch for that freeware site but it does have an adventure section.
Would Oregon Trail qualify? There’s a website that emulates a Mac, www.virtua lapple.org/oregontraildisk.html link broken because I don’t know if it’s considered pirated and/or against board rules
I’ve never heard of this game before but it looks pretty good
An Untitled Story is very good. A metroidvania style exploration game with some pretty neat platforming. The easiest difficulty is very easy, the hardest very hard, I’d recommend playing on normal. Get it from the creator’s site, it’s the middle one in the second row (I’d link straight to the game but can’t because of crappy web design).
Or how about Within a Deep Forest? You play as a bouncy ball, and have to switch between materials with different physical properties to defeat the platforming challenges. Lots of physics fun.
Another, much smaller game I like is Eversion. To quote, “a cute platformer with a dark secret”. Good for a morning or afternoon of fun, but perhaps not an evening or night.
It’s by the guy who did Dangerous Dave so if you liked that…
Tower of Heaven was recently featured on a gaming blog I read. It’s a fun little platformer with hilarious rules.