Guest posts - Guest writing - Which sites?

It depends on what niche you want to write about. You can search from Technorati or Google, you can install SEO Quake to see pagerank, links, etc.

Business and law mostly. I have seen so many wiki sites that just accept any old spun rubbish. In the long run I question whether they will last, so I am hoping to find sites that actually care about teh quality of the writing.

Search google for blogs or sites looking for guest posts by using specific search terms like: 'inurl:guest-blogging-guidelines'
or 'inurl:guest bloggers'

Also, just try more general searches like 'write for us' or 'submit guest post'. To narrow it down to sites related to your niche, add your main keywords into the search, for example 'write for us dog training'

Yeah, it depends a lot on the niche that you want to write about.
You can do what val said to find blogs that allow guest posts.

The best way to find blogs accepting guest posts in your niche is to join guest blogging communities, lately many new sites come up but the biggest and the ones where you can find many guest post opportunities are:
Guest blogging: Looking for guest bloggers or guest post? Join MyBlogGuest!

and

http://www.bloggerlinkup.com/

Both of them are great, myblogguest works as a forum where bloggers who need guest posts post a thread and you can find them. On the other hand bloggerlinkup send a newsletter 2-3 times per week where you can find many bloggers asking for guest posts or you can make your own offer of guest posts...

Try both of them, you will easily find what you need...

I've had my best luck using specific search terms, as Val suggested.
I use a very general keyword that might be a category or the main keyword of the type of site I want. Then I add "write for us", "submit guest post", "guest author", "guest post" or any other phrases I see people use on their blogs if they take guest posts.

I then look at how many comments those blogs get. If it's one where there's very little interaction, I might take note of the blog, but I don't necessarily submit a post. The idea is to get as much traffic as possible out of each post.

Next I look at the topics covered in the blog. Are there ones that are relevant to my market and my site? If the readers of that blog aren't going to be interested in my products, there's no point spending my time writing.

I've used My Guest Blog and Blogger Linkup in the past, but I've found that there generally aren't that many blogs that have a lot of interaction on their sites. I haven't checked those sites recently, so maybe that's changed. However, if you're just in it to get some links, vs. traffic, then it doesn't matter as much.

Ralf and kochtgr both took the words out of my mouth Both of those services are great (and free! How can you beat that?!)

Another thing I've done is to search for a site that I like that takes guest posts. Then, I hit the "pages similar to (whatever I searched for)" and see what else pops up. It's a great way to find sites that you didn't necessarily know about -- but are great resources.

Dude, just use the following Google search commands to find blogs for whom you can make guest posts based on specific keywords:

inpostauthor:guest + keyword
inpostauthor:"guest blog" + keyword
inpostauthor:"guest post" + keyword
keyword + intitle:"write for us"
keyword + intitle:"contribute to"
keyword + intitle:"submit" + inurl:blog

These commands would give you all blogs that accept guest posts from anyone for whatever keyword you type in and, thus, the better your keyword research is... the better this technique will work.

Of course, there is more to guest blogging than "open to the public" sites: there are tons of website owners who have a blog and are looking for good content to put on there and ways to promote their own content. In other words, you guest post for them and they'll do it for you. Fair deal. These guys and girls are only a Tweet, message or mail away.

My opinion? Use both techniques and see what you can come up with. The second techniques DOES require you to actually have good content on your site before a website owner will even consider accepting a guest post... but it'll definitely be worth your while!