Absolutely anything can be a suitable subject for a blog entry; a Website of interest, OpenSource software, breaking news, a personal anecdote, a witty comment from a colleague, politics, a joke, a holiday, Microsoft, music, gadgets, your teams latest victory, anything.
Just because someone else (an esteeemed blogger or maybe even a professional journalist) has written an article on the same subject, don’t necessarily let that put you off. Lots of people are searching blogs for a personal view and not that of a journalist. For example, if I am considering the purchase of a digital radio for my car, I would rather hear your views on the same make and model rather than read a two year old review in AutoExpress.
As for technical content, while there might be reams of manuals, a wealth of text books and assorted wizards engaged in long running, complicated discussions with religous fervour that simply overwhelm you, remember that there will always be newcomers struggling in silence to insert their first row into their first table. So, if you have just written a squash ladder in PHP and mySQL, go ahead and share your knowledge.
If you can’t think of anything to say, say nothing. Don’t be afraid of radio silence. Personally, I don’t bother with sites offering solutions for writers block such as ‘99 subjects to blog about’ as I am not really interested in your response to ‘If you were an animal, what would you be and why ?’ [Although curiously enough I was asked exactly this question at interview once !]
Think about your favourite Web sites and blogs and why they are of particular interest to you. If you love reading about long haired Persian cats, and you have a long haired Persian cat, then write about that as it’s likely that this will appeal to a group of people with similar interests.
Don’t get discouraged because no-one comments on your blog or nominates you for ‘Top 100 Blogs’. Think of all the blogs you have read in the last week with interesting, valuable content where you never had the common courtesy to take the time to say ‘Thanks’.
Finally, think carefully about any brilliant ideas that surface while under the influence. Your audience may enjoy it immensely but you might live to regret it.