It seems that we've had rain everyday in August here in the UK. Fortunately the temperatures haven't dipped too, otherwise there would be a danger of tomato blight.
One effect that rain has on tomato plants and their fruit is that it dilutes the taste. You'll get a more intense taste from a tomato whose plant has been slightly under-watered.
Tomato split is also the result of either over-watering or too much rain. This happens when the inside of the tomato swells faster than the outside skin is able to cope with and the skin splits.
I cut my split tomatoes along the line of the split and nobody knows the difference!
If growing tomatoes in a greenhouse, wet weather may affect your plants by condensation and humidity. Too much of either can cause fungal diseases (tomato blight again) and cause skin blemishes like ghost spot. This happens when grey mould spores fall onto the skins and leave small white rings.
Thanks to Pete E. for the photo.