You asked for it, you've got it!
Add your useful weblinks here... if I get time and it seems productive, maybe I'll paste all the websites into this top post, to prevent it running on too long.
Brief rules: no personal sites please, no ads, no spamming, start another thread if you want to discuss any of these sites. (Again, if it seems productive, perhaps I can append a link to the discussion thread next to the site link.)
Recommended sites so far (thanks everyone):
http://www.alcs.co.uk/ - The Authors' Licensing and Collecting Society
http://www.plr.uk.com/ - Public Lending Right re books borrowed from UK libraries. Dont forget to register your book!
Markets and opportunities
http://pred-ed.com/ - Preditors and Editors, so you can check out that literary agency or publisher to make sure they're not rip off merchants.
http://www.womagwriter.blogspot.com/ - Very useful for guidelines and submission details for fiction in women's magazines.
http://www.bbc.co.uk/writersroom/ - BBC Writers Room
http://www.angiebenedetti.com/blog/category/anthology-market-listings/ - Constantly updated list of anthologies and submission requirements/payments details.
http://www.duotrope.com - The Duotrope Digest is a searchable writers' resource of over 3200 pro- to non-paying publications which accept submissions. It is updated almost daily and you can subscribe to feeds.
http://www.magazinesabout.co.uk - Magazine and periodicals directory (UK)
http://www.writingcalendar.com/index.htm - Sally Quilford's calendar of forthcoming writing competitions.
http://prizemagic.co.uk/ - lists and lists of competitions to enter
Advice and blogs on the publishing world:
http://deborah-durbin.blogspot.com - blog on making a living out of freelance writing.
http://howpublishingreallyworks.com/ - Does what it says on the tin.
http://www.alanrinzler.com/blog/ - The Book Deal, an inside view of publishing by veteran editor Alan Rinzler
http://www.karenmiller.net/index.cfm?page=1 - About Writing - lots of great advice from author Karen Miller
http://pimpmynovel.blogspot.com/ - insider insight and advice on publishing world
http://www.annemini.com/ - Author! Author! This one is like taking regular exercise, excruciating and necessary...
http://www.advicetowriters.com/ - A new inspirational or practical quote from a wide selection of writers every day.
http://thebookshelfmuse.blogspot.com/ - If your mind needs stirring this blog has an: Emotion Thesaurus; Setting Thesaurus;Color, Textures and Shapes Thesaurus; Symbolism Thesaurus.
Software for writers:
http://www.openoffice.org/ - Open Office word processing
http://celtx.com/ - scriptwriting
http://www.spacejock.com/yWriter5.html - novel writing
http://www.waterstones.com/waterstonesweb/ - To check if Waterstones stock copies of your book(s).
http://localbookshops.tbpcontrol.co.uk/tbp.web/customeraccesscontrol/home.aspx?d=localbookshops&s=C&r=10000020&ui=0&bc=0 - To search for books and local bookshops and order from them.
http://photobucket.com/ - For loading and editing your photos for your website or blog. You can even keep your albums private if you want.
http://www.phrases.org.uk/ - for useful phrases and proverbs, including the Bible, Shakespeare; and American, French and Latin.