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Some things I've worked on: http://linkedin.com http://rapportive.com http://rescuejs.com
I'll start the ball rolling:
1. Linode - its flexibility has helped us keep costs seriously low without sacrificing power or scalability.
2. Memcached - using it various places instead of database access helped us turn a beast of a ~12second algorithm into a sub-400ms beauty.
I suspected they'd both be pretty cool but didn't realise quite how insanely fast memcache would be.