While the original version of the Overwatch Workshop was a little bit bare-bones, Blizzard has slowly but surely transformed it into something wonderful. Best of all, they're still frequently issuing new updates for it - always adding new ways to script events and add features, all of which then result in even more complex game modes.
With the Workshop's one year anniversary now behind us, Blizzard has taken the time to highlight some of the most popular, user-made modes currently available on the Workshop. Here's the full list, as well as the code you'll need in order to play them:
1v1 Arena — TXCXX
Spider Widow — 5D4FX
Zombies — YNZQS, S35Z1, 2QBQN, MGJ03
Thomas — 321Z9
Practice Range 2.2 — AJERA
Dodge the Cars — PAGTH
Clone Wars — 0QJFH
Aim Trainer — KAVE5
Boss Fight — Z0TV6
Pictionary — FH93A
Uno — 44H7K
OWWare — QDCRY
Scrim Lobby — TEYSH
Widow 1v1 — XQK96
Ana Paintball — 1DMTZ
While most of these make perfect sense as far as custom game modes for an FPS go, I have to question the sanity of people that have a fun game like Overwatch in front of them, but instead choose to play Uno!
On a less silly note, the above-mentioned practice modes are well worth the trouble, and will genuinely help your aim out if you stick with them for a little while. So if you're not happy with the state your gameplay is currently in, don't fret as it's entirely possible to fix with a little bit of effort.
Have fun with the game modes, good luck with any sort of training you might do, and you can follow any future developer comments over at the Overwatch forums.