I was looking around for some alternatives to Basecamp, and was pretty surprised to see that there are a ton of alternatives out there. So naturally a list is the way forward.
No recommendations yet, and not all of these are Basecamp replacments - Harvest and ClockingIt, for example, could be seen as complimentary.
- Goplan
- Unfuddle
- OnStage
- Harvest (time tracking and invoicing)
- TeamWorkLive
- ActionThis
- 5pm
- DeskAway
- Collabtrak
- ActiveCollab (self hosted)
- ProjectPier (fork of older version of ActiveCollab, selfhosted, opensource)
- ClockingIt (timetracking, opensource)
- Lighthouse
- Copper
- Wrike
- Intervals
- Central Desktop
- Huddle
Technorati Tags: basecamp, projectmanagement

If you need to send project summaries or project wrap-ups to clients from Basecamp, check out http://www.exportreports.com.
Its a new service, currently in beta which allows you to export reports from Basecamp to PDF(with beta pricing starting at $0.99 per report).
Hope you find it useful (and if you have any suggestions, please let us know!).
One more for your list: Intervals. It is a web-based project management service that is somewhere between Harvest and Wrike. Focusing on time and task tracking, it also handles documents, light invoicing, and reports.
And another - CentralDesktop is great. Have been using them for over a year.
Anu - don't forget about home grown http://www.huddle.net/
Collaboration tools are like mobile phones, lots of different options and every group has different needs. For those working with less sophisticated groups of users (i.e. non-techies), check out ProjectSpaces. It is yet another great basecamp alternative that focuses on usability and caters to less tech savvy users.