Depends what you define as internet speed. I used to use a small tool called bwm. It just showed simple output of the send and receive rate in bytes/sec for each network interface.
The following tool goes a little further and shows transfer speed for each open connection. It works for X and console mode using ncurses.
http://freshmeat.net/projects/connmon
Ntop is also shows a lot of network statistics but I've never used it. From what I've read it's pretty extensive in what it can monitor and the output it can produce
http://www.ntop.org
