Let tomatoes simmer slowly for hours.
Pros: probably the tastiest solution.
Cons: time consuming, energy (gas/power) consuming, loss of "fresh" tomato flavor, if you want that.
Add flour/bread/carrots/potatoes/whatever to add consistency.
Pros: quick and easy.
Cons: may change the flavour. May require heating to near-boil to activate the starch.
Remove seeds and gel/pulp with fingers or a spoon.
Pros: quick, and removes seeds, which may give a bitter taste.
Cons: removes pulp (which gives most of the 'umami' flavor) and juice.
Use a strainer to remove the watery part.
Cons: removes part of the juice.
Drain (without seeding) tomatoes before to bake them - a (very) light salting helps to draw out excess liquid.
Pros: Removing liquid before allows for less cooking time and preservation of fresh tomato flavor
Cons: waiting time, and removes part of the juice.
Halve and place on a sheet pan, then bake until sufficiently dewatered for your purposes. May have to leave oven door cracked open.
Pros : Easy. Skins are easy to peel after roasting. Adds some caramel/roasted flavors where the sugars burned.
Cons : Not quick. Not good if you want skins on. Loses the 'fresh tomato' flavor.
Bake as you would to create a sun dried tomato (at low heat) but don't put it in olive oil. Can reduce the time to not remove as much moisture.
Pros : Easy.
Cons : Very slow (we're talking most of the day slow). Loss of the fresh tomato flavor.
Use a plum tomato, or other variant that has a higher flesh to gel ratio.
Pros : Extremely easy (if you can get them at your market)
Cons : Often have less flavor than beefsteak, heirloom and other varieties (but may not be noticeable if you're buying from a grocery store)