Sql Server Management Studio for 2008 Express?
If you are licensed to use the full Management Studio, I would install that. It has more features that will come in handy if you ever need to manage other SQL Server instances other than your local development/Express version. If you're not licensed or you know this is not a requirement, you can download Management Studio Express from here:

http://msdn.microsoft.com/en-us/library/ms365247.aspx

Yes, that's for 2008 R2; probably better to get the most recent release, which will work just fine with 2008.

Whether you end up using the full version or the Express version, can't hurt to install Service Pack 1 once SSMS is installed:

http://www.microsoft.com/download/en/details.aspx?id=26727

(Probably more appropriate for the full version, since there is a greater chance that issues are fixed since it includes more features.)