Forgetful
Every single one of those, in the past week. ...wow, that's kinda sad, but then, I DO need to invest in a new computer.
This is an extremely good idea, and not just because you can mess things up without realising it.
I once lost a few hours of work because I found out too late that Hammer was saving only about half of the VMF file to disk, and when it finally totally crashed it couldn't read back the nonsense it had created. (And yes, the automatic .vmx backup file it makes was mauled too.)
So backups are your friend 
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
The only time Hammer crashes for me is when I try to run Portal 2 through Hammer using "Run Map." I got around it by exiting Hammer, exiting Authoring Tools, wait for a minute, then launch Portal 2.
My Work
[spoiler]Maps:
Revenge of the Angry Turrets
Capture the Cube [Co-op]
Capture the Cube 2 [Co-op]
TPWEGTH Sample Map
Aperture Aquatic Testing Center
Aperture Aquatic Testing Center 2
Aperture Time Testing Center
ML's Halloween Trick - 1000 downloads!
ML's Halloween Treat
ML's Combination - 1000 downloads!
ML's Jailbreak Labyrinth
ML's Tricky Teamwork [Co-op]
WIP:
"Capture the Cube 3"
Workshop Maps Link: http://steamcommunity.com/profiles/76561198008890579/myworkshopfiles/[/spoiler]
I always uncheck the option to run map after compiling.
This problem dates back to the Half-Life 2 mapping days.
Vengeance
Olympic
[SP] Interception
Interception PeTI
Herman
Uprise
Unified
Same here. For me it just begun very early. Maybe someone remembers to Hammer in 2005. It crashed more than now in Portal 2.