Found several custom maps so far. The maps are decent for them not having the tool they are going to release soon. Coming from a legit website, I would imagine it is safe.
This one is self explanatory. Read the .txt that comes with it. All you do is double click the "Install" folder, and copy/cut the first portal folder you see. Paste it in your C:\..../steamapps/"YOURUSERNAME" folder that contains the folder portal.
http://halflife2.filefront.com/file/Portal_SS_Orange_Box_Map;83692
This next one only comes with a map file so just copy/cut and paste it into your C:\.../steamapps/"USERNAME"/portal/portal/maps folder
http://halflife2.filefront.com/file/Portal_Facility_Map;83668
There are others but they are from some random forums.
~Pitlith
Edit: To load the first one called "SS_Orangebox", load Portal, then select New Game and go all the way to the right. It will be next to "Level 19".
To load the second one called "por_facility_v100", you have to type into your console "map por_facility_v100". That will load it.
This one is self explanatory. Read the .txt that comes with it. All you do is double click the "Install" folder, and copy/cut the first portal folder you see. Paste it in your C:\..../steamapps/"YOURUSERNAME" folder that contains the folder portal.
http://halflife2.filefront.com/file/Portal_SS_Orange_Box_Map;83692
This next one only comes with a map file so just copy/cut and paste it into your C:\.../steamapps/"USERNAME"/portal/portal/maps folder
http://halflife2.filefront.com/file/Portal_Facility_Map;83668
There are others but they are from some random forums.
~Pitlith
Edit: To load the first one called "SS_Orangebox", load Portal, then select New Game and go all the way to the right. It will be next to "Level 19".
To load the second one called "por_facility_v100", you have to type into your console "map por_facility_v100". That will load it.