I was going to recommend Monsoon Flat Panel computer speakers. I mastered 3 CDs on these to great result. The speakers cost me $170 and were absolutely great for what I needed. However, Monsoon speakers are no more. Here is an interesting little article on them:
http://en.wikipedia.org/wiki/Monsoon_speakers

Now I have 8" Rokit speakers. They make a 5-inch model that has been very popular for computer edit stations on a budget. Here's a review on the 8s and 5s:
http://www.soundonsound.com/sos/aug0...s/krkrokit.htm

Here they are for sale, but the 5s are $150 each.

Event speakers have a couple of good low cost models, but not at $100.

I believe for a powered decent low cost reference monitor that's worth buying, $300 is about the bottom limit for a pair.

The speakers above for $100 that need an amp would be the best way to go if you have an amp around to power them.

It's possible to get used speakers fairly cheap if you are in an area with a lot of budding musicians.

I agree that if you don't have headphones, that's a better investment than cheap speakers with a hyped sound. The headphones will have a use for years.

However, as pointed out, the headphones are not ideal for mixing sound, but they are better than cheap speakers that will give you a totally false impression of the actual sound.

As pointed out above, there are different ways to go about referencing your sound. When I need music for something that will be on the internet, even if I already have the music, I go to the music site and listen to the sample on my computer speakers to get an idea how it will sound to other people. Some music that sounds great on speakers sounds terrible over the computer speakers.

Now that I think about it, maybe if you are doing video for iPods, headphones are the perfect way to work on the audio since most everybody will be listening with headphones.

Yeah, don't get headphones alone. Get some cheap speakers too.

Use studio-quality headphones for EQ'ing. Use the speakers for mixing subtle sounds to the right level. And use the speakers for setting the L/R balance. And make sure the result sounds good on both outputs.

Headphones can make subtle details very apparent. They will be lost in the mix in a typical environment. And headphones don't convey phase information properly. Always check the L/R balance with speakers, unless the target listener will be using cans as well.

I fully agree with Michael and Jon. Subtile mixes sound really different on headphones.

We mostly use Sennheiser headphones during initial editing (not to disturb the others in the same room) and Behringer b2031 speakers on final soundsweetening.

Heh yea, I`ve already stated I have Sennheiser headphones, I use them in the field, but don`t want to use them to mix.

I will likely end up getting some M-series monitors, my friend works at Guitar Center so I can get a good deal on them. Thanks for pointing me the way of them.

I think getting the best speakers you can afford is the way to go, and just spend time cross referencing them to 'learn' what they hype or lack, then mix accordingly, and cross reference again. And make sure the listening environment is treated.
After all, most engineers check their mixes in their car stereos. It ain't the arrows, it's the archer...