High Scalability Building bigger, faster, more reliable websites.

Update 6: Things We’ve Learned at 37Signals. Themes: less is more; don't worry be happy.
Update 5: Nuts & Bolts: HAproxy . Nice explanation (post, screencast) by Mark Imbriaco of why HAProxy (load balancing proxy server) is their favorite (fast, efficient, graceful configuration, queues requests when Mongrels are busy) for spreading dynamic content between Apache web servers and Mongrel application servers.
Update 4: O'Rielly's Tim O'Brien interviews David Hansson, Rails creator and 37signals partner. Says BaseCamp scales horizontally on the application and web tier. Scales up for the database, using one "big ass" 128GB machine. Says: As technology moves on, hardware gets cheaper and cheaper. In my mind, you don't want to shard unless you positively have to, sort of a last resort approach.
Update 3: The need for speed: Making Basecamp faster. Pages now load twice as fast, cut CPU usage by a third and database time by about half. Results achieved by: Analysis, Caching, MySQL optimizations, Hardware upgrades.
Update 2: customer support is handled in real-time using Campfire.
Update: highly useful information on creating a customer billing system.

In the giving spirit of Christmas the folks at 37signals have shared a bit about how their system works. 37signals is most famous for loosing Ruby on Rails into the world and they've use RoR to make their very popular Basecamp, Highrise, Backpack, and Campfire products. RoR takes a lot of heat for being a performance dog, but 37signals seems to handle a lot of traffic with relatively normal sounding resources. This is just an initial data dump, they promise to add more details later. As they add more I'll update it here.

Adapted From the website:

HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for web sites crawling under very high loads while needing persistence or Layer7 processing. Supporting tens of thousands of connections is clearly realistic with todays hardware. Its mode of operation makes its integration into existing architectures very easy and riskless, while still offering the possibility not to expose fragile web servers to the Net.