Allegro NFS Server — source code is open source; prebuilt binary is gratis for non-commercial usage.
http://opensource.franz.com/nfs/
http://www.nfsforwindows.com/home

Crossmeta NFS Services from Pavitrasoft — proprietary.
http://www.pavitrasoft.com/products/crossmeta/

Cygwin NFS Server and Client — open source under GPLv2.
http://ftp.uni-kl.de/pub/windows/cygwin/release/nfs-server/
http://cygwin.com/

DiskAccess (client) and DiskShare (server) by Shaffer Solutions Corp. LLC (formerly from Intergraph Corp.) — proprietary.
http://www.ssc-corp.com/nfs/products.asp

Distinct NFS Server by Distinct Corp. — proprietary.
http://www.distinct-it.com/

InterDrive Client 4.0 for Win95/NT by NetManage, Inc. (formerly FTP Software) — proprietary.
http://www.netmanage.com/

Microsoft Services for Unix (formerly Interix from Softway Systems, Inc.) — proprietary: gratis download, for MS-Passport users).
http://microsoft.com/sfu/

NFS for SmarTerm from Esker Software (formerly Persoft) — proprietary, apparently discontinued.
http://www.esker.com/

nfsAxe from LabF (gratis download, proprietary, US $40 to use long-term).
http://labf.com/nfsaxe/

NFS Maestro by Hummingbird Connectivity Technologies, Inc. — proprietary.
http://www.hummingbird.com/

Omni-NFS by XLink Technology, Inc. — proprietary.
http://www.c2000.com/products/csp_omni.htm

PathWay Client NFS by Attachmate Corp. — proprietary, apparently discontinued.
http://www.attachmate.com/

PC-Xware by PCD — proprietary.
http://www.ncd.com/products/software/pcxware/ppcxds.html

ProNFS by Labtam Inc. — proprietary.
http://www.labtam-inc.com/

Reflection NFS Client by WRQ. — proprietary.
http://www.wrq.com/products/reflection/nfs_client/

Rumba Office 95/NT for TCP/IP by NetManage, Inc. (formerly Wall Data) — proprietary, apparently discontinued.
http://www.netmanage.com/

Solstice Network Client for Windows from Sun Microsystems, Inc. — proprietary, discontinued; firm recommends WRQ's Reflection NFS Client, instead).
http://www.sun.com/software/solstice/netclient/

SOSS-NFS (Son of Stan's Server NFS) — open source under GPLv1.
http://www.loa.espci.fr/winnt/sossnt4/sossnt4.htm
http://dl.winsite.com/files/87/ar2/winnt/netutil/sossntr4.zip
http://support.jgaa.com/index.php?MenuPage=download

SuperNFS by Frontier Technologies Corp. — proprietary.
http://www.frontiertech.com/Product/SuperNFS/product_details.asp

Term Professional by Century Software — proprietary, apparently discontinued.
http://www.censoft.com/

War NFS Daemon by Jarle Aase (now unmaintained) — proprietary, gratis usage for non-commercial purposes.
http://www.novatech.net/jgaa/warnfsd.htm

Alternative: Use Samba. (Most people trying to do NFS on MS-Windows are solving the wrong problem.)