I was in a group with one remote player for a little over two years, and have also taken part in multiple long-term online-only campaigns using various tabletops.
I don't really disagree with anything Dorian said, but I'm posting this to elaborate on how we did it, and explain my experience with the tools we used.
Please note up front -- if you are playing a game that has no need of a map or a grid, but which rather relies on theatre of the mind, Skype (or your preferred voice chat client) is pretty much all you need, but that should be fairly obvious, so I suspect, given that you're asking, that that's not the case.
So, if you are using a map/grid, shift the map to be online, but keep as many players local as you can
The specific software you use isn't too important. Our remote player happened to be a developer highly involved in the maptools framework community, so we used maptools for the tabletop, and ventrillo for voice.
As time went on, we shifted more and more onto the maptools framework. Character tokens, complete with power macros, which rolled dice and automatically damaged and applied the correct status icons to the correct targets. This made the experience seamless between the local and remote players, but isn't strictly necessary, and regardless of which platform you do it on, this level of automation requires a lot of coding (both up-front and any time a character levels up, dies, retrains, or gains new items), so is not necessarily ideal for all groups.
To keep the experience as tangible and social as possible, the local players gathered in the living room of the DM, with maptools running on his large TV, and the players passing around a wireless mouse based on initiative order. The DM was simultaneously logged in from a laptop, which he used to control the monsters, maps, and adjudicate status effects.
I have also participated in online campaigns that use skype and teamspeak in conjunction with Maptools, Roll20, FantasyGrounds, and the defunct 4e online table. Maptools remains my favored tabletop software, due to how extensible it is once you learn its scripting language. I don't think that there's a noticeable enough difference in the quality level of the voice chat options for it to matter whatsoever, but clarity is absolutely important, lest the remote player/s feel left out.
If you use a power based system on a macro-based online tabletop, it is pretty important that players have updated, printed out copies of their character sheets, or at least some form of their full power cards handy and accessible. Especially as characters level up, the number of power options becomes fairly overwhelming, and the specifics of how an effect triggers, or its range, or duration are easily forgotten. It's usually possible to click edit on the macros to see the full text in pseudo-code, but it's much easier to look at your character sheet and try and make decisions about what to use before your turn.
Finally, if you use a power macro-based online tabletop, I highly recommend that at least the DM make a point of becoming highly proficient in the scripting native to that tabletop's macros. Even if you allow each player to be responsible for their own character's macros, some players macros will inevitably be better than others -- and formatting their output in a standardized fashion is more conducive to quick understanding of what's happening than having them be all over the place.
The level of automation that these online tabletops are designed for is overkill for many of the simpler systems - but the ability to keep a segmented repository of easily shareable maps, images, and sound effects can still be useful, and if your system uses dice rolls, the ability to remotely computerize them ensures that the remote player is not tempted to cheat.
Feel free to comment with any follow-up questions and I will edit them into place.