I love shooting on overcast days. You basically have one giant soft fill light almost anywhere you shoot outside. Bring along a speedlite with an umbrella or small soft box and it's easy to introduce a key light and get some nice soft portraits. It's also nice not to have to compete with the strong sun light, it becomes a lot easier to overpower the sun with your speedlite.
I actually want to get around to hauling my Elinchrom strobes outside this summer. The difficulty has been the lack of power, or rather having no place to plug your strobes into when shooting outside. This had been one of the key reasons I have stuck with the speedlites for the time being. However, a friend of mine recently bought the Alienbee battery pack which supposedly is able to also run some of the Elinchrom strobes.
On the flip side, reflectors and diffusion panels are awesome to have on bright sunny days. I want to get more into that but I'll leave that for another entry.
In the end, these small accessories that help you control light will give you a lot more freedom in the conditions you can shoot in. They're fairly cheap and you can get a lot of use out of them. Knowing how to control light to your benefit will benefit you a lot more than a sharper lens or a better camera body. Invest in those!