I want to know that which wordpress plugin must be used for seo. Doing seo is very difficult. And seo servvices is costly. So is there any plugin which can optimize the wordpress blog?

Well, you can try All in one SEO pack wordpress plugin. It doesn't automatically do SEO for you but it makes it easier to add tags for your posts and your pages. You may also want to add plugins like sitemap generators so it wouldn't be much of a drag for you to create it again and again or everytime you have an update on your site.

I personally use Wordpress SEO by Yoast on all my self-hosted Wordpress blogs and it has been awesome for me!

I don't use this one, but I know it does have a very good reputation.

Doing SEO in Wordpress is the EASIEST thing ever. Who told you it's hard?

Without needing any additionals plugins you only have to fill the tags of posts. That's how Google indexes your page. Thats the single most important thing you must do (and most people don't).

Want to get better results? Install for free:
- http://wordpress.org/extend/plugins/seo-ultimate/
- http://wordpress.org/extend/plugins/tagpages/

Commercial solutions:
- http://scribeseo.com/

However without tags, that's not going to matter.

Google loves WordPress. I've sometimes posted a topic and found it indexed within just a few minutes.

i am trying that now.