It really depends on how and what are you managing.

For large-scale projects, Atlassian's Jira & Confluence can be really good, but they can be customised (in a quite hard way) to be useful for smaller teams as well.

If you are looking for an easy to use & instant solution with very slim learning curve, probably one of these will fit for your need: https://asana.com/http://www.pivotaltracker.com/https://podio.comhttps://basecamp.com/ (It's more like a forum as George mentioned)

My best tip is to check all of them, see what can they provide and go with the one your are falling in love.