Free plan isn't displayed, but available at
https://signup.37signals.com/basecamp/Free/signup/new
The current signup urls are:
Maximum: https://signup.37signals.com/basecamp/Max/signup/new
Premium: https://signup.37signals.com/basecamp/Premium/signup/new
Plus: https://signup.37signals.com/basecamp/Plus/signup/new
Old Plans:
Basic: https://signup.37signals.com/basecamp/Basic/signup/new