I'm sure it will work on Dead island, I've tried it. Dunno about riptide, I don't own it

check cooptimus

The trick should essentially be starting the game twice and making a lan game.

There is games that support local coop out of the box, Slam Bolt Scrappers, Forced, etc. Check the steam store for more.

Risk of Rain and Spelunky are two I've tried recently.

Left 4 Dead 1 had a way. I remember playing with my brother in local coop, me on keyboard/mouse and him with a controller.
You had to setup some config files, probably best to google it cause it was a long time ago and I can't remember how to do it anymore.

Pretty much any LEGO games support local co-op.

Portal 2

Fifa 14 , Mortal Kombat , Street Fighter , GRID 2 , Flatout all of them ( party mode rulez xD )

Resident Evil 6 has splitscreen co-op

jamestown does local co-op and is pretty awesome.

Worms also does local co-op, if you're into those games =D. Shank & Shank 2 as well, though you may need a controller. Also trine and... well... clicky... clicky...

Step 1. Get Deathspank
Step 2. Play local co-op and dispense JUSTICE
Step 3. ????
Step 4. Profit.

How to Survive.

castle crashers

Dungeon Defenders.

Risk of Rain, just about any of the LEGO games, Portal 2, that's all I can think of.

capsized

Dead Pixels

Downtown Run, Worms, Trine, Heroes, Bionic Commando

Portal 2 works. All my friends finished the co-op first because I wanted to play single player first.
It's awesome to play co-op with yourself because you can take your own time with no pressure :)
http://forums.steampowered.com/forums/showthread.php?t=1862192

Heroes of might and magic 3. Best hotseat game of all-time, easily.

Sonic & All-stars Racing transformed supports up to 4 players in split-screen and is a ton of fun, especially if you like arcade kart racers :) PC version has several bonus characters from PC-only games including TF2 cast.

If you liked metalslug or the kinds, you'll like mercenary kings. Really fun in coop, even tough split screen is a bit awkward in 2 player(you'll have the screen divided in 4 nonetheless).