There are two tools that I had never used before in the garden that now I simply could not live without. First is the "ken ho" weeder. I have 2 now, one is from the Asian grocery store and it works very well, and only cost about six dollars, but it isn't stainless. The other is stainless a bit smaller but works just as well. These suckers can weed! They are especially helpful on things like ground ivy and not bad on crabgrass either.
The second tool is a real workhorse, the garden knife or soil knife. It makes dividing perennials much easier and for really big weed roots it can't be beat.
Two more essentials that you may not think are useful in the garden shed are a pair of scissors, and a pair of pliers. The scissors cut twine, shade cloth and all sorts of things. The pliers pull out nails that seem to be stuck in fences and pieces of wood, weeds emerging from cracks in pavement, and even stakes that seem to have been cemented into the ground.