I need a "total nerd" category. Ahem.
We've been visiting other planets for almost 50 years now! But we've visited some much more than others. Mostly, that's because some planets are much easier to reach than others... Mars and Venus are closer both in distance and in delta-v (basically, the energy a spacecraft needs to get there). The outer planets are very far off; Mercury is so deep down the Sun's gravity well that it's very hard to reach.
Just for the hell of it, I did a tally.
I'm breaking these down into flybys (which just blow past a planet) and "dedicated missions" (which actually go into orbit or land). Flybys are cheaper and easier, so there are more of them. Note that this means some missions get counted more than once, since some spacecraft have done flybys of more than one planet.
- Flybys - 1 (Mariner 10, 1974-5)
- Dedicated mission - 1 planned (MESSENGER, arriving 2011)
Venus: Damn, we've sent a lot of stuff to Venus. Though it tapered off sharply after the wave of Soviet probes in the 1960s and '70s.
- Flybys - 5
- Dedicated missions -- 21, of which 16 were Soviet. Most of the Soviet ones were pretty short-lived. The only currently active probe is the ESA's Venus Express, in orbit since 2006 and still transmitting.
Mars: If we count successful missions (with "success" meaning at least a few seconds of data sent back) Mars is actually just behind Venus. Of course, the successful Mars missions have tended to last a lot longer. Since Phoenix died a few weeks ago, there are five missions currently active at Mars: three orbiters, and the Spirit and Opportunity probes rolling around on the surface.
- Flybys - 6
- Dedicated missions - 18
- Flybys - 7
- Dedicated missions - 1 (Galileo, in orbit 1995-2003). The JUNO mission is expected to launch in 2011 and arrive in 2016.
- Flybys - 3
- Dedicated missions -1 (Cassini, in orbit since 2004)
- Flybys - 1 (Voyager 2, 1986). No future missions currently planned.
- Flybys - 1 (Voyager 2, 1989). A Neptune Orbiter mission is under consideration, though it wouldn't arrive at the planet until at least the late 2020s.
- Flybys - 1 planned (New Horizons, arriving 2015)
What's interesting (to me, anyway) is the generational timescale. Jupiter had one orbiter arrive in 1995; the next will come in 2016 at the earliest. Mercury, over 30 years between visits. (We still don't have a complete map of Mercury at a decent resolution.) Uranus and Neptune... phew. There's a decent chance of a Neptune flyby in the 2020s (it would slilngshot past the planet to visit the Kuiper Belt), but we might none of us live to see a Uranus orbiter.
On the plus side, we'll still be getting interesting news from other planets... oh, long into my childrens' lifetimes. There are kids in kindergarten today who are going to run planetary science missions in the 2040s. I'm okay with that.