I want to create a blog site in SP 2007 that allows the users to vote on existing blog posts but not to their own. Provide users instant voting talleys next to each post. How do I go about in accomplishing this is SharePoint 2007.
One of the possible solutions is :
1) Create a list 'Votes' to hold votes with a look-up column 'Post Id'. This column will fetch the look up values from 'Id' column in Posts list. Also, create 'Vote'(Yes/No) and 'Voted by'(Person) columns.
2) Create a custom control which will have just a button to vote. You can hide the button from current user if he is the author of current post. For e.g if(SPContext.Current.Web.CurrentUser.Name==SPContext.Current.File.Item["Author"].toString()) //don't render the button.
3) On 'Vote' button click, you can check(by using CAML query) in the 'Votes' list if the current user has already voted against the current post(Post Id). If not, you can add his vote as new list item along with 'Post Id'.
This doesn't address the "but not their own" component of your question but I've found that when content authors know that votes aren't anonymous the practice of "voting up" their own posts becomes self-regulating.
UPDATED- Forgot to mention that the solution above does have a configuration switch to prevent multiple ratings, so even if your content authors try to upvote their own posts they can only do it once.