Many complaints have come in from Android users about limited memory space. After installing just a few of their favorite applications, they had little memory left. This is because prior to 2.1 you were not able to save your applications on your memory card. This will all change once Android 2.2 is released, but that leaves many of us stuck with low storage since not everyone will get the update. Apps2SD is a program that will allow you to move your software to your memory card thus freeing up valuable space. Please do not do this with less than a class 4 SD card as it may eject and you will have a whole other set of issues. It is recommended to get a class 6 SD card if you are wanting to try this. At the present time there is no official support for this but you will be able to install this on most android devices. Make sure to back up everything because doing this will wipe out your SD card. To see a complete set of instructions, go here http://wiki.cyanogenmod.com/index.php/Apps2SD
If you want to avoid this trouble, clear some cache out of some of the programs you are running and also check your application list and delete any unused programs. Every little bit helps!