validating only a part of a web Page

This can be implemented by using validation groups.

Enterprise Level Deployment: Features and solutions

Ted Pattison says "packaging and deploying your WSS development efforts using solution packages is a best practice, and knowing how to do this should be considered an essential skill." 

There are numerous ways to package   SharePoint solutions, from manual creation of manifest.xml and diamond directive files (DDFs), to Visual Studio projects that use the Windows SharePoint Services 3.0 Tools: Visual Studio 2005/2008 Extensions (VSeWSS).

LIst of custom log viewer

• SPLogViewer - supports loading and filtering one file, does not follow/tail log displays data in a DataGrid.
• SharePoint Log Viewer  
• Stonebond SharePoint Log Viewer  
• ULS Deobfuscator - 
• SharePoint ULS Log Parser - one click access to log files, supports filtering by Process Name, Level, Area and Category.
• Log Viewer Feature - access from Central Administration, select individual log basic filtering on category, event and trace severity, output as HTML in Central Administration.
• WSS / MOSS Log File Reader - Access to individual log files from Central Administration, standard DataGrid filtering and sorting options.
• Stefan Gordon's ULS Viewer - Open individual log, does not follow/tail log, displays data in ListView detail. Filter by secerity, process category and free form text.
• SQL Integration Services - All reporting, sorting and filtering options provided by MSSQL (Blog Post with ETL Package Download), time consuming to setup.
• LogParser - Familiar interface to IIS Administrators, pleanty of options. Setup takes too long for small installations.

is there a way to view who is requesting access to a site?

Yes you recieve a email for request but there is no form to see it. At most you can configure the email for same Site Actions > Site Settings > Advanced Permissions (under "Users and Permissions" group ) > Settings > Access Requests

Assign permissions to library view

Sometime a solution came to have views with different column for different set of user. No its not possible. 

How to chnage application pool that a web app is using.

No simpler out of box solution. need to use powershell
http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=276

Pointing Your SharePoint Servers at Themselves

 make sure that every server in the SharePoint farm points to itself for all web apps. If  something goes wrong we can  use RDP to log in to each server and try to pull up SharePoint. If this attempt works, then I know that the server is working

Installing a PDF iFilter

PDF files will be indexed and can be searched. one of the best usage of search
http://www.foxitsoftware.com/

SharePoint doesn't tell IIS about post webapp creation.

Be aware if you create an Alternate Access Mapping (AAM) for a web app in Central Administration, you still need to add the host header for the new address  into IIS   .

Should we use virtual SQL server for SharePoint?

RAM, CPU, or drive space all are restricted in Virtual enviornment
Everything in SharePoint is stored in SQL Server, if SQL Server is slow, SharePoint is slow.
This should be last option. Recommended in physical box. if you have to create a virtual sql server plese see following video http://technet.microsoft.com/en-us/sqlserver/gg429826.aspx

log of login attempts to sharepoint 2007?

I am still searching for it and couldnt find. one solution may be to create list and then update list through user control embedded in master page.

Minimum RAM or Hard Disk Space requirement

Moss2007 requires following minimum prerequisite for performance

https://technet.microsoft.com/en-us/library/cc262485(v=office.12).aspx