GAOSHAN  This is nothing more than a front-end for the unix command bzip2 (which is painfully simple to use without paying a penny):

To compress a file:

1. open Terminal

2. type: bzip2

3. drag the file to be compressed and drop it onto Terminal

4. Hit the return key

To decompress a file:

1. open Terminal

2. type: bunzip2

3. drag the file to be decompressed and drop it onto Terminal

4. Hit the return key

If you are seriously low in memory, for step 2 type: bzip2 -s

To vary the level of compression, for step 2 type:

bzip2 -1

bzip2 -2

" "

bzip2 -9

these set the level of compression. -9 is used by default (i.e. if you don't set anything, that means -9 will be used) and, frankly, the best choice. The other settings don't really have much affect worth noting.

Never pay for an application that does not offer any significant convenience or ability.  
(Version 1.0.2)