I have a grassy weed with flowering seedheads at the top. What is it and how do I get rid of it in my yard?
That is annual bluegrass, Poa annua. It grows in lawns during the spring (and/or fall, depending on climate) and causes some very nice lawns to thin out, as it grows very thick and chokes out the lawn grass. The problem with removing it is that this is a grass itself, so you cannot kill it with selective weed killer. This site gives good information on the management of the grass. Do not overwater your lawn, especially in shady areas. When you water the lawn, water deeply and infrequently, as this grass is a shallow rooted water addict. Do not overdo the fertilizer as that will give these a head start over the turf. Keep your soil loose, as this species likes compacted soil. Maintaining a thick, vigorous lawn will discourage the grass from spreading.
Also, if you use PGRs (plant growth regulator), such as flurprimidol (a-(1-methylethyl)-a-[4-(trifluoromethoxy)-phenyl]-5-pyrimidinemethanol) which slows the growth of the Poa annua more than the growth of the perennial turf grasses, it give these good grasses a head start, as well as drastically reducing the Poa annua's ability to produce seed heads, which will lead to decreasing populations of this weed in lawns each year that this treatment is applied.
You have Wintergrass. There are selective pre-emergent herbicides to treat it. Otherwise, your best bet is to pull out the individual clumps as soon as you see them as they seed extremely fast. They will pull out with ease if you get them early enough.
Product example: http://www.bunnings.com.au/amgrow-100ml-winter-grass-killer_p2980004
The method I used to remove it from my lawn was to put a drop of non-selective herbicide at the base of each plant. It was a lengthy process but it doesn't take much poison to kill this weed.