Every other week I stumble across web-sites that prove invaluable. At the time, I’m left wondering how I could have missed them for so long, and once noticed they seem to pop up everywhere. Here are 5 sites I use on a regular basis. Each one has had a major impact on my online activities:

1. iStockphoto - Massive resource of photos from amateur and professional photographers. Before finding this site, sourcing images for use on web-sites and blogs was always a problem. Do you have rights to use the image? Should you just ‘borrow‘ it and say nothing? Most of the images sell for $1, and are royalty free. Once you set up an account, downloading the odd image is a one click process.
2. Bloglines - I have a list of about 30 blogs I watch regularly. Until I started using Bloglines to keep me updated on changes, I had to click on each bookmarked link every time I wanted to see if a new post had been submitted. The beauty of using an online service like Bloglines, is that you can access these favourite blogs from any PC, and you’re immediately notified not just of new posts, but changes to existing posts as well - always interesting to see how other people edit their work. Try it out, and don’t forget to add me to your list.
3. Copyscape - Have you ever had your work copied and posted somewhere else? Copyscape is a simple site that scans the web for complete or partial copies of anything you post on your blog. It’s useful to track down plagiarisers, and other writers who are quoting and commenting on what you’ve written. The first time I used it I found this - a word for word copy of one of my own posts.
4. Squidoo - The premise behind Squidoo is that search engines now return so many results for any query that they are fast becoming meaningless. Far better to have a single page written by an ‘expert‘ on a particular topic with further links to articles, books, blogs, and other resources. Anyone can be an expert and create a lens on any topic, from links about your own product to resources for eBay sellers.
5. Ping-o-Matic - Most blogging environments like Wordpress automatically ping all the relevant sites after you add or change a post, but every now and then the ping seems to get lost along the way, and sites like Technorati fail to display your stunning new post. Entering your URL into Ping-o-Matic allows you to send the pings yourself, at any time.

What else am I missing out on?