I personally try not to buy anything for HubPages, since my income is not very big, but once I make lots of money and need to reduce the tax costs, I would make sure I have:
1. A word processing program that checks for spelling and grammar. Besides writing hubs, you can use it to save your research notes. I would get the whole Microsoft Office suite, so that I could also use the spreadsheet to keep track of my income and changes I made to hubs, etc. and have Powerpoint to create slideshows I could use videos.
2. A photo processing program that would help me show my photos at their finest.
This isn't software, but some other investments:
1. Quality computer with lots of memory
2. Fast internet speed
3. Comfortable chair (and ergonomic setup.)
4. Quality camera - a small one that I can carry around with me all the time so I can get those photos