Take a look at some features of the Syn-O-Matic
SEO WordPress Plugin :
1 New synonymizing algorithm never released before! (now you can use two-word phrases synonymizing)
2 Existing posts rewrite option.You can set,rewrite or not rewrite
all existing posts made earlier.
3 HUGE synonym base/library included! (we provide synonym base/library with more than 50,000 words !)
4 You can use single synonym table/base for many blogs.You can store and manage synonym base/library at your remote server.It's a very useful feature.You can create only one synonym base, on a remote server and then set unlimited plugins at several other domains to connect to this base.
5 Rewriting posts on fly! (lower server load and optimal execution time ) Very efficient if you have a lot of blogs/splogs!
6 Regulated rewrite level. You can set the percent of words that should be rewrited.
7 You can create your own original synonym bases/libraries.
8 Special tag for no rewrite.You can put it in to every place you need, and marked posts/titles will not be rewrited.
9 Different type of rewrite modes.
10 Easy 3-step installation and easy interface use!
11 Free lifetime updates and support !
12 Etc ,etc
You can purchase it here
http://www.professional-submitter.com/prices.html just for 49.90 ! (unlimited blog license!)