If you like tinkering then the Seeedstudio Stalker is worth a look - basically a data logging Arduino. There are other Arduino solutions too including nanode and various shields. You can then take your choice of RH/Temp sensors for the job in hand.

For a plug and play solution take a look at TinyTag.

As I'm currently finding out, RH sensing isn't as straightforward as it seems, unless you want to spend a lot of money on sensors.

I've bought three different sensors so far. One reads RH accurately (it's laser trimmed and calibrated) as a voltage, which is fine except it won't drive long cables directly; it costs around £20. One will drive long cables and outputs a frequency proportional to RH, but needs some calibration constants read out from a chip before first use (offset and gain, in essence) and these then need to be accounted for in the logger somewhere; it costs around £17. Finally I've been looking at using raw, non-calibrated capacitive RH sensors and making up tiny boards to read the capacitance change, perform the required calibration calculations and output RH in a meaningful and accurate data steam that will work over long cables. The raw sensors are less than £5, but the small ucontroller to do the calibration and data transmission stuff is another £2.50 or so, plus I've only started playing with it this afternoon, so I'm not absolutely certain I can get it to work properly.

The latter looks as if it might work, and accurately calibrating RH sensors looks easy enough. 0% can be obtained by sticking the sensor in the space above a sealed jar of good desiccant and 75% RH can be obtained by doing the same except using a thin layer of moist salt in place of the desiccant in the jar.

As for data logging, then as others have suggested there is a wealth of DIY type solutions around but they all need a fair bit of work to get them to do something useful. The logger I've been working on will record RH as well as temperature, and is in this thread: http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8133&page=2#Item_4

There is a dearth of affordable logging and sensing units around, AFAICS, unless you don't mind paying out £100 or more for a setup, or unless you only want to monitor a single point.

Never used the holder, look at the www.homechip.co.uk website. You can just try and attached 2 wires to it and then attach them (the right way around) to the reader. Works for me

May give that a go and compare it to my other sensors. I like tabletop science.

Then wash fingers for an hour.
Off to the beach now.