Hi nicksteel,
I don't know about freeware tools probably cool edit if you can still find it but for me I have bought a commercial software to do this for me.
I use Magix Audio Cleaning Lab and the results I receive from Tape to Mp3 are fastastic, once you record your audio it has a audio cleaning wizard and a mastering wizard (If you wanted to use that also) to clean up the hissing sound.
You can of course manually set your filtes up if you prefer.
It will save it as wav or mp3.
I have converted a few tapes that I have with this tool and im well impressed.
I have the 2004 version but the new 2005 version is only £19.99 a real bargain if you ask me.
You can get the 2005 version
>Here<
It's just an option im sure others may be able to point you to free tools.
jorel suggested a few good tools in the link to the post below, one being RIP Vinyl a free trial is available of this.
http://www.kvcd.net/forum/viewtopic.php?t=770