A good way of getting software cheap are the MacHeist bundles - basically they promote software by giving them away at over 90% off

Have a look at the currently running
MacHeist 3


Top Mac Software that I recommend:


Bookends


EndNote was the undisputed king under MacOS 8 and 9 for everyone who had to write a thesis or publication. But when MacOS X took over EndNote became bloated, slow, buggy, and crashed more often than I care to remember.

Bookends is what EndNote should have been - a must for everyone citing literary sources.


RapidWeaver


iWeb is a first class program included in iLife - unfortunately it is rather feature limited. And more powerful programs like DreamWeaver cost an arm and a leg.
RapidWeaver steps into that breach - it has power to spare but is as easy to use as iWeb. And a bargain, especially when bought as part of a bundle (look out for MacHeist).


Intaglio


I loved ClarisDraw - clear interface, did everything I needed, and very easy and comfortable to use. Intaglio is a worthy successor on MacOS X, and it can open my old ClarisDraw files. Wonderful.

OmniGraffle


OmniGraffle is maybe the best program I ever encountered - version 5 propelled it into a class of its own. Try it just to be amazed at how a Mac program should be (Microsoft, are you listening?).