UPDATED
With scanning and repair tools.
More to come, but these cover most needs. …

UNDELETE

Piriform Recuva
https://www.ccleaner.com/recuva/builds

EaseUS® Data Recovery Wizard
http://www.easeus.com/datarecoverywizard/free-data-recovery-software.htm

Glary Undelete
http://www.glarysoft.com/products/utilities/glary-undelete/

Free Undelete (OfficeRecovery)
http://www.officerecovery.com

Handy Recovery (30 day trial)
http://www.handyrecovery.com

PC Inspector
http://www.pcinspector.de/Default.htm?language=1

PhotoRec, Digital Picture and File Recovery
http://www.cgsecurity.org/wiki/PhotoRec

FreeUndelete (NTFS)
http://ntfsundelete.com
http://blog.atola.com/undeleting-files/

EXTRACT FROM DAMAGED OPTICAL DRIVES

Roadkil’s Undelete and Unstoppable Copier (useful for damaged data CD/DVDs)
http://www.roadkil.net/listing.php/C1/Data%20Recovery

Exact Audio Copy (rip or re-make a scratched Audio-CD)
http://www.exactaudiocopy.de

INFO, SCAN, FIX and Salvage
SpinRite – (The best there is, but no free version)
https://www.grc.com/spinrite.htm

HDD Regenerator (commercial product, but will fix 1 block at a time for free)
http://www.dposoft.net/hdd.html

DiskFresh
http://www.puransoftware.com/DiskFresh.html

TestDisk
https://www.cgsecurity.org/wiki/TestDisk

HDDScan – free HDD diagnostic utility
http://hddscan.com

RS Software Lab, Disk Checker
http://www.rssoftlab.com/diskchecker.php

CheckDisk by Dirk Paehl, and GUI by Emiel Wieldraaijer
http://www.paehl.de/cms/checkdisk
http://www.wieldraaijer.nl

MHDD by Dmitry Postrigan (low-level diagnostics and repair)
http://hddguru.com/software/2005.10.02-MHDD/

Miscellaneous Disk tools at HDD Guru.
http://hddguru.com/software/

Ultimate Boot CD (many tools in one bootable ISO)
http://www.ultimatebootcd.com

Many excellent tools from MiniTool (even for Macintosh)
http://www.minitool.ca

Many tools from Active@/NTFS.com
http://www.ntfs.com/products.htm

Victoria HDD Utility
http://alternativeto.net/software/victoria/

S.M.A.R.T. Monitoring
HDD Guardian
https://www.majorgeeks.com/files/details/hdd_guardian.html

CrystalDiskInfo
http://crystalmark.info/?lang=en

HDD Health
http://panterasoft.com/hdd-health/

Acronis Drive Monitor
http://www.acronis.com/en-gb/personal/hard-drive-health/

Active@ DiskMonitorFree
http://www.disk-monitor.com

S.M.A.R.T. Monitoring Tools
http://sourceforge.net/projects/smartmontools/
GSmartControl
http://gsmartcontrol.sourceforge.net

SmartCheck
http://www.urltoy.com/psc.htm

PassMark DiskCheckup
http://www.passmark.com/products/diskcheckup.htm

Hard Drive Monitor
http://www.harddrivemonitor.com

Defraggler (inc. SMART tab)
https://www.piriform.com/defraggler/builds

—-> HDD FAQ <—-

Fix Corrupt Files
File Repair
http://www.filerepair1.com

All Media Fixer
http://www.realconvert.com

 

Free Tools lists

http://www.forensicswiki.org/wiki/Tools:Data_Recovery
http://www.techrepublic.com/blog/five-apps/five-free-disaster-recovery-tools

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TIPS:

The drive got very slow to boot Windows over the days/weeks and now the drive/partition cannot be read or says is unformatted

Sounds like it was struggling to read blocks, but was not retiring them.
Usually these are blocks near the begining of the drive.
It really needs a Low level format to start with fresh blocks again.

I assume you are now booting from another drive.
As long as Windows can see the device rather than the contents, a SMART tool will report the current condition (and show history if available).
HD Guardian is the best, but can be heavy on CPU, and uses .NET4.
It is very complete and can enable all disabled SMART modes.
https://dr-flay.vivaldi.net/s-m-a-r-t-tools/
There are also links to some manufacturer tools there.

You have several choices.

First it may be worth booting the “Ultimate Boot CD” and running Partition Magic.
This boots to a mini Linux distro with disk fixing and preparation tools.
Sometimes its file-manager can still read the partition and let you in. If it takes ages to list, then it is hitting the bad blocks in the file allocation table.
However you may be able to copy the files you wish to save before repartitioning and full formatting the drive.

Alternatively, boot Ultimate Boot CD and run the drive manufacturer DOS tool, See if you can enable the offline data collection and test modes.
Do a quick SMART test. (2-3 mins max) This checks for errors and should be able to list them.
A long test will take about an hour or more, but will try to move the data from the bad blocks.

The 2 tools that would do the best “fix in place” job are both commercial.
SpinRite and HDD Regenerator (you can use HDD Regen to display all the errors and delays on the drive. If you only have 1 bad block, the free demo is all you need !).
MHDD is free and can display and fix blocks, but is a bit confusing unless you are used to DOS programs.
Disk Fresh is free and only runs in Windows, but does not show much feedback, and you can’t see where the bad blocks are.
If you use Disk Fresh, use the RAW device mode if you can.

Chances are, if you check with HDD Regenerator or MHDD you will see a cluster of bad blocks near the beginning. If they are dotted all over the drive, you must do a complete wipe and format. If you just get a cluster, you can just fix those blocks.
If they are right at the beginning of the drive, you also have the option to leave a space before the partition and ignore that area.